Petroleum is the primary source for liquid and gaseous fuels. It has long been a concern that the availability of petroleum will decline because known petroleum reserves are being consumed and exploration for new reserves is becoming increasingly difficult. Since so many technologies rely on liquid and gaseous fuels, the need to develop processes to produce such fuels from alternative sources is widely recognized.
Several technologies have been developed to address this concern, many focusing on carbonaceous solids, such as coal, petroleum coke, and even organic wastes. Processes have been developed to convert these materials into various combustible gases such as “syngas,” a mixture of carbon monoxide and hydrogen, or methane, also known as synthetic natural gas.
The gasification of coal is typically achieved by reacting steam and coal at very high temperature, or at moderate temperatures in the presence of alkali metal catalysts. One such process, to catalytically convert coal to methane, is disclosed in U.S. Pat. No. 4,094,650, which issued on Jun. 13, 1978. The '650 patent discloses that in the presence of a carbon-alkali metal catalyst, carbonaceous solids can be gasified to produce methane in a relatively thermoneutral process.
The alkali metals, including lithium, sodium, potassium, rubidium and cesium, can catalyze coal gasification, either in pure metallic, or compound, or complex form. Generally, the relative activity of the alkali metals as coal gasification catalysts increases with atomic weight, i.e., cesium is the most potent and lithium is the least potent. The development of catalytic coal gasification (“CCG”) processes has focused on sodium and potassium since they exhibit reasonable activity while also being less expensive and more widely available than the heavier alkali metals. The coal-catalyst mixture can be prepared and then introduced into a gasification reactor, or can be formed in situ by introducing alkali metal catalyst and carbonaceous particles separately into a reactor.
The alkali metal catalyst can be introduced into a CCG process as an inorganic alkali metal salt, an organic alkali metal salt, an alkali metal hydroxide, an alkali metal oxide, an alkali metal carbonate, an alkali metal bicarbonate, etc., or as a pure metal, or as a mixture of such compounds. The alkali metal catalyst can comprise more than one alkali metal, e.g., potassium and sodium, and can be introduced as a combination of alkali metal compounds, e.g., a mixture of potassium hydroxide, potassium carbonate, and sodium hydroxide, which combination may be eutectic salt mixtures. One preferred alkali metal compound identified in the literature for use in CCG is potassium carbonate.
Coal typically contains significant quantities of inorganic matter including calcium, aluminum, silicon, iron, vanadium, and sulfur, among others. These compounds form inorganic oxides or ash in the gasification reactor. It is known that at temperatures above about 500 or 600° C., potassium (or other alkali metals) can react with the ash to form insoluble alkali aluminosilicates. In this form the alkali metal is inactive or relatively inactive as a catalyst. To prevent a buildup of the inorganic solids in a coal gasification reactor, a solid purge of char, i.e. solids composed of the ash, unconverted carbonaceous material, and alkali bound within the solids, must be periodically withdrawn. This char can be 20% or more by weight, e.g., of the potassium metal, including some as soluble potassium salts such as K2CO3, and some as insoluble potassium aluminosilicate such as KAlSiO4 (synthetic kaliophilite or kaolinite).
To compensate for losses of catalyst in the solid purge, a traditional CCG process uses a substantial catalyst make-up stream. Raw material costs and environmental implications of a CCG process can be minimized by recovering the alkali metal from the solid purge.
The '650 patent discloses water leaching of the solid purge to recover the soluble portion of the alkali metal. The solids from the gasifier are cooled to 700° F., and then mixed in a rich aqueous solution of the catalyst to dissolve readily soluble material. The enriched aqueous catalyst solution is utilized to prepare the gasifier feed. The once-washed solids are transferred to a multi-stage countercurrent liquid solid extraction system wherein the solids are contacted serially with an increasingly dilute catalyst solution at about 110° C. (230° F.) and 30 psia to recover the less soluble alkali material. The '650 patent discloses a 7-tank battery of mixing vessels with attendant filters, pumps, and make up streams. Insoluble alkali metal compounds such as KAlSiO4 remains in the spent solids, so only about ⅔ of the alkali metal withdrawn in the solid purge can be recovered and recycled to the gasification process.
It is known that insoluble alkali constituents can be recovered from aluminosilicate compounds by “digesting” alkali aluminosilicates in an alkaline solution containing calcium or magnesium, which under proper conditions displace the potassium (or other alkali metals) from the aluminosilicates and form an aqueous solution containing freed potassium (or other alkali metals).
Such a process is utilized in U.S. Pat. No. 4,159,195, which issued on Jun. 26, 1979. According to the '195 patent, the solid purge and solids separated from the raw product gases pass through a fluidizing chamber to separate out and recycle the lighter particles to the gasifier. The remaining heavier particle stream is cooled and directed to a water leaching unit wherein soluble alkali constituents dissolve to form a dilute alkali solution containing a variety of alkali metal compounds such as carbonates and sulfates. The '195 patent states that the water leaching unit typically comprises a multi-stage counter current extraction system, suggesting a complicated process similar to that disclosed by the '650.
The '195 patent warns that the soluble alkali constituents could react with the alkaline components of the digestion process and form undesirable byproducts. To avoid such problems, the '195 patent directs the dilute alkali solution to the gasifier feed preparation zone without further treatment. The leached solids are directed to a lime digestion unit wherein the washed solids are vigorously mixed with aqueous slurry of calcium or magnesium hydroxide at between 250-500° F. (˜120-260° C.). According to the '195 patent, the digester product solution normally comprises alkali metal hydroxides and alkali metal aluminates. To avoid recycling aluminates to the gasifier which could increase the ash load, the '195 patent discloses that the digester solution can be contacted with a carbon dioxide containing gas causing the aluminum hydroxide to precipitate before the remaining solution is recycled.
A multi-stage water-wash process with digestion is conventional. The '195 patent reports that greater potassium recovery (than with just a water-wash process) is possible by lime digestion to recover the insoluble moieties. However, pilot scale work on catalyst recovery, sponsored by DOE contract ET-78-C-01-2777 and reported in FE-2777-31, which utilized digestion followed by a multi-stage leaching with water, shows that the addition of lime digestion may not be economically advantageous.
Lime digestion has also been suggested for use in other coal technologies. For example, magnetohydrodynamic (MHD) power generation has been proposed as a technique to increase the efficiency of a conventional coal-fired power plant. In an MHD power plant, a plasma, formed by adding an easily ionizable material to the combustion gases, passes at very high temperature and high velocity through a magnetic field and induces an electric current. According to U.S. Pat. No. 5,057,294 to Sheth et al., which describes a process for recovering and recycling spent seed in an MHD plant, potassium carbonate is a preferred seed material.
According to the '294 patent, most of the potassium seed converts to solid potassium sulfate upon reaction with sulfur dioxide in the combustion gases and can be subsequently recovered as potassium formate by reaction with lime and carbon monoxide in a formate reactor. About 15% of the potassium seed reacts with the aluminum and silicon inorganic components of the coal and forms insoluble potassium aluminosilicates known as MHD “slag.” The potassium in the slag can be substantially recovered by crushing the slag particles and digesting them with lime in aqueous phase at a 4:1 molar OH/K ratio and a temperature of 445° F. to 480° F. (about 230-250° C.). The resulting highly alkaline digester product solution of potassium hydroxide is combined with the formate reactor product to precipitate calcium sulfate. The remaining solution with potassium formate and small amounts of potassium hydroxide may be dried and recycled as MHD “seed.”
The '294 patent discloses that by grinding the slag to about 75 mesh before it is digested, up to about 80% of the potassium can be recovered. The slag and spent seed can be collected as substantially separate streams. The '294 patent does not address recovery of soluble and insoluble alkali from a single stream of solids. There is no suggestion that the particles could be fractured rather than ground to properly size the particles for digestion.
Thus, the known methods for alkali metal recovery in coal processes are cumbersome and expensive. It would be highly desirable to develop a CCG process capable of recovering alkali catalyst in a simpler system, and it would be even more desirable if such a process were flexible and reliable.